Scientific Method —

Bubble, bubble, toil and trouble

High speed film gives an interesting view of droplets impacting on a surface.

Bubbles and droplets form an important part of modern life.From your household ink jet printer to agricultural spraying and painting, the behavior of droplets plays an important role in the performance of these tasks.Of particular importance is the behavior of the droplet when it impacts upon a surface.For instance, ink from an ink jet printer should stay exactly where it was placed, not splash and preferably not spread either.The problem is complex, depending on the surface tension of the drop, the viscosity of the fluid, and the inertia (mass and speed) of the drop.Our understanding of how all these parameters interact to cause of drop behavior as it impacts a surface is very primitive, however, some new research (summary here) is shedding light on the topic.

Researchers in France and the Netherlands have filmed droplets made from a water-glycerol mixture falling on a hydrophobic surface.They found that under certain conditions a thin, high velocity jet of liquid was ejected from the drop on impact and that significant air/fluid mixing resulted.These jets are extremely quick and short-lived, necessitating that the researchers film at 60,000 – 100,000 fps.Further investigation revealed that the droplet doesn’t flatten like a pancake.Instead the hydrophobic surface encourages the drop to form a suction cup shape, trapping air between the drop and the surface.The air cavity eventually collapses, causing a jet to shoot out of the drop at velocities up to 20 m/s (the impact velocity is only 0.5 m/s).In many cases the remaining fluid entrains the air pocket as bubbles.

This research may turn out to contribute significantly to understanding the performance limitations of various coating processes such as ink jet printing and spray painting.

Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com//Twitter@exMamaku